1. Field of the Invention
The present invention relates to a color image processing apparatus for processing color image data to alter characteristic of the color image into a different characteristic.
2. Description of the Prior Art
A digital color image processing apparatus has been known in which a specific color of a portion of an original is converted into a desired color in real time. In this known apparatus, colors are judged as being the same either on condition that the color component ratios of these colors are equal or that the color component densities of these colors are equal. This color conversion apparatus is disclosed, for example, in the specification of the U.S. Pat. No. 4,204,728 assigned to the present assignee. The present applicant also has proposed, in the specification of U.S. patent application Ser. No. 084,012, a device which enables a color to be converted into a variety of colors. An improvement in this device is disclosed in the specification of U.S. patent application Ser. No. 120,820.
In the known art in which the detection of a color is conducted on the basis of the color component density data, if the range of criteria for the density data is selected to be too narrow, there is a risk that two colors of the same hue are judged as being different colors due to the difference in the color component density data. In particular, in case of an image which is obtained through, for example, an image scanner, any edge portion between a region of a specific color and the white blank cannot be judged as being of this specific color, because in such an edge the density gradually changes though the hue is the same. On the other hand, if the range of criteria is selected to be too wide, colors of different hues may erroneously be judged as being the same color if the color judgment is conducted on the basis of the color component density data. Such an erroneous detection may occur particularly when a color component takes the maximum value in the range of criteria while another color takes the minimum value of the range. The color judgment of the known art relying upon the color component ratio involves a problem in that the S/N ratio is so small that the color component ratio is seriously affected by noise to cause erroneous detection, particularly when the value of the input color image is small.
It is assumed here that a predetermined color has input values of R=200, G=100 and B=100, i.e., a color component ratio of R:G: B=2:1:1. In such a case, a color represented by R=2, G=1 and B=1 also is detected as being the predetermined color. Practically, however, an error on the order of .+-.2 inevitably occurs due to influence of noise. In consequence, the reliability of color detection is impaired particularly in the region where the density level is low.
In recent years, digital color copying machines have become popular. In a known digital color copying machine, a color original image is color-separated and digitally read. Image signals thus read are subjected to an image processing process which may include color correction and gradation correction, and the processed signals are delivered as image data to a color printer, whereby a color copy image is obtained. In this type of color copying machine, the gradation and density of the color image are determined by the values of color component data of the respective colors read from the original. On the other hand, in order to obtain a color copy image with a high degree of fidelty, it is necessary that the data concerning the color components constituting the color image, i.e., yellow, magenta, cyan and black, are processed in a well balanced condition so as to ensure a proper color balance. To comply with such a demand, correction is conducted by using, for example, a look-up table (LUT) such that appropriate balance of color components such as yellow, magneta, cyan and black is maintained over the entire density range, as illustrated in FIG. 61(a). When it is required to increase or decrease the image density over the whole area of the image, the characteristics of the LUTs of the respective colors are shifted. For instance, when it is required to increase the density, the characteristics of the LUTs are shifted in the direction A as shown in FIG. 61(b), whereas, when it is required to decrease the density, the LUT characteristics are shifted in the direction of the arrow B. Thus, conventional density control has relied upon simultaneous shift of characteristics of LUTs of all colors.
A discussion will be provided hereafter as to a case where a demand exists for a change in the density of a whole image having density characteristics as shown in FIG. 61(c). It is assumed here that a point on the original has color component data (Y.sub.1, M.sub.1, C.sub.1) and the inputs for the respective color components are changed by an equal amount. In response to such a change in the input, the image data is changed from (Y.sub.1, M.sub.1, C.sub.1) to (Y.sub.1 +y, M.sub.1 +m, C.sub.1 +c).
The values y, m and c are different from one another as will be seen from FIG. 61(c) so that the balance between the color components such as yellow, magneta and cyan is undesirably changed as a result of the change in the density.
Thus, in the conventional system which makes use of independent conversion tables for the respective color components, a change in the density undesirably causes a change in the color, particularly when linear conversion characteristics are used. For instance, even in the case of an original of gray color, the color balance deviates from that of the original gray color, failing to reproduce the original gray color with a high degree of fidelty.